There has been substantial work directed towards development of implantable defibrillation systems that avoid the necessity of a thoracotomy. Systems that deliver a defibrillation pulse between one or more endocardial electrodes and an active defibrillator housing are disclosed in U.S. Pat. Nos. 4,727,877 issued to Kallok; 4,922,927 to Fine et al.; 5,129,394 to Mehra; 5,133,353 to Hauser; 5,261,400 to Bardy; and 5,374,279 to Duffin, Jr. et at., all of which are incorporated herein by reference. As used herein, the words "housing", "enclosure", "case", and "can" are synonymous.
If a device is chosen to have an active can and is placed pectorally, it is unlikely that this decision would be changed in the future. If a lead is used subcutaneously and the inactive can implanted abdominally, it is again unlikely that there would be reason to change this, to "activate the can". Even if this decision were reversed, a surgical procedure would likely be required to reposition the can for effective use as an electrode. Therefore, external programmability of this choice of active or inactive can is not needed, allowing the design and manufacture to be less complex.
Thus, while it is desirable to provide a choice of having the device housing function as an active electrode, this choice will almost always be made at the time of device implant, avoiding the added device complexity resulting from having the choice be programmable after implant.
In the invention of Pless et al., U.S. patent application Ser. No. 08/221,811, filed Mar. 31, 1994, now abandoned which is assigned to the assignee of the present invention, a cardiac defibrillator with a case that can be electrically active or inactive is disclosed. A special connector cavity is provided that has one terminal electrically connected to the generator case and a second terminal connected to one pole of the defibrillator output. By plugging in a pin long enough to contact both terminals, the case is activated during a defibrillation shock. No means is disclosed for using a standard connector pin, such as a DF-1 defibrillator connector pin. Because of the desire to be compatible with existing lead connectors without requiring a special adapter, both at initial implant and during a pulse generator replacement, it would be useful if the pulse generator case could be activated while allowing the use of standard lead connectors. The device disclosed in U.S. Pat. No. 5,374,279 to Duffin, Jr. et at. has a similar deficiency in that a special lead connector or plug must be provided to activate the defibrillator case.
In U.S. Pat. No. 4,301,805 to Peers-Trevarton et al., a cardiac pacer connector system is disclosed that is provided with a bridging means for using the pacer case as a reference potential when a unipolar lead is used. When a bipolar pacing/sensing lead is used, the bridging means is not operated, and the pacer case is inactive.